So far we have addressed only sentences which are statements. Lojban, like all human languages, needs also to deal with sentences which are questions. There are many ways of asking questions in Lojban, but some of these (like questions about quantity, tense, and emotion) are discussed in other chapters.

The simplest kind of question is of the type “Is it true that ...” where some statement follows. This type is called a “truth question”, and can be represented in English by Example 14.92:

Note the two formulations. English truth questions can always be formed by prefixing “Is is true that” to the beginning of a statement; there is also usually a more idiomatic way involving putting the verb before its subject. “Is Fido a dog?” is the truth question corresponding to “Fido is a dog”. In Lojban, the equivalent mechanism is to prefix the cmavo xu (of selma'o UI) to the statement:

Example 14.92 and Example 14.93 are equivalent in meaning.

A truth question can be answered “yes” or “no”, depending on the truth or falsity, respectively, of the underlying statement. The standard way of saying “yes” in Lojban is go'i and of saying “no” is nago'i. (The reasons for this rule are explained in Section 7.6.) In answer to Example 14.93, the possible answers are:

and

Some English questions seemingly have the same form as the truth questions so far discussed. Consider

Superficially, Example 14.96 seems like a truth question with the underlying statement:

By translating Example 14.97 into Lojban and prefixing xu to signal a truth question, we get:

Given that Fido really is either a dog or a cat, the appropriate answer would be go'i; if Fido were a fish, the appropriate answer would be nago'i.

But that is not what an English-speaker who utters Example 14.96 is asking! The true significance of Example 14.96 is that the speaker desires to know the truth value of either of the two underlying bridi (it is presupposed that only one is true).

Lojban has an elegant mechanism for rendering this kind of question which is very unlike that used in English. Instead of asking about the truth value of the connected bridi, Lojban users ask about the truth function which connects them. This is done by using a special question cmavo: there is one of these for each of the logical connective selma'o, as shown by the following table:

(This list unfortunately departs from the pretty regularity of the other cmavo for logical connection. The two-syllable selma'o, GIhA and GUhA, make use of the cmavo ending in “-i” which is not used for a truth function, but gi and i were not available, and different cmavo had to be chosen. This table must simply be memorized, like most other non-connective cmavo assignments.)

One correct translation of Example 14.96 employs a question gihek:

Here are some plausible answers:

Example 14.103 is correct but uncooperative.

As usual, Lojban questions are answered by filling in the blank left by the question. Here the blank is a logical connective, and therefore it is grammatical in Lojban to utter a bare logical connective without anything for it to connect.

The answer gi'e, meaning that Alice is a dog and is a cat, is impossible in the real world, but for:

the answer e, meaning that I want both, is perfectly plausible, if not necessarily polite.

The forethought questions ge'i and gu'i are used like the others, but ambiguity forbids the use of isolated forethought connectives as answers – they sound like the start of forethought-connected bridi. So although Example 14.105 is the forethought version of Example 14.104:

the answer must be in afterthought form.

There are natural languages, notably Chinese, which employ the Lojbanic form of connective question. The Chinese sentence

means “Do you walk or run?”, and is exactly parallel to the Lojban:

However, Chinese does not use logical connectives in the reply to such a question, so the resemblance, though striking, is superficial.

Truth questions may be used in bridi connection. This form of sentence is perfectly legitimate, and can be interpreted by using the convention that a truth question is true if the answer is “yes” and false if the answer is no. Analogously, an imperative sentence (involving the special pro-sumti ko, which means “you” but marks the sentence as a command) is true if the command is obeyed, and false otherwise. A request of Abraham Lincoln's may be translated thus:

In logical terms, however, “but” is the same as “and”; the difference is that the sentence after a “but” is felt to be in tension or opposition to the sentence before it. Lojban represents this distinction by adding the discursive cmavo ku'i (of selma'o UI), which is explained in Section 13.12, to the logical .ije.)